[ShadeTreeMech]

I think a factor that may be throwing our figuring and pondering into the unknown is the flux of the actual mix of e85.

"E85 is an alcohol fuel mixture that typically contains a mixture of up to 85% denatured fuel ethanol and gasoline or other hydrocarbon (HC) by volume. On an undenatured basis, the ethanol component ranges from 70% to 83%."

WIthout a static ratio of dino fuel to corn fuel, it is difficult to nail down any figures concerning the stuff. My only suggestion is to use the price of the e85 compared to the normal 87 octane to determine what the target mpg would be to get the same miles per dollar or better with the mix. In my case I used a bit of algebra to determine that, for the price, if I got 17 mpg or better on the e85 compared to the price at the same station of 87 octane, i would come out ahead on my cost per mile, which I did in a non e85 vehicle AKA the 97 mercury villager with fuel injection.

I don't claim to know much as it concerns e85 vs 87 octane, only personal experience. But there seems to be more mpg available per unit of energy in e85 than in 87 octane.